This invention relates to pressure-sensitive adhesives. More particularly, this invention relates to pressure-sensitive adhesives having a high refractive index.
Pressure-sensitive adhesives (xe2x80x9cPSAsxe2x80x9d) are defined herein as adhesives which exhibit permanent tack at room temperature. This property allows pressure-sensitive adhesives to adhere tenaciously upon application with only light finger pressure. PSAs have a balance of properties: adhesion, cohesion, stretchiness, and elasticity. Adhesion refers both to immediate adhesion to a surface and to the bond strength which develops upon application of pressure (often measured as xe2x80x9cpeel strengthxe2x80x9d). Cohesion refers to the xe2x80x9cshear strengthxe2x80x9d or resistance of the applied PSA to failure when subjected to shearing forces. Stretchiness refers to the ability to elongate under low stresses. Elasticity refers to a property wherein the material exhibits a retractive force when stretched and retracts when the force is released.
Pressure-sensitive adhesives have many diverse applications including applications in optical products. For certain optical applications, it is useful to match the refractive index (RI) of the adhesive to that of the substrate to which it is applied. This matching of refractive index enhances the optical properties of the construction by reducing glare and reflectance. Glare is defined herein as the average reflectance over a range of 450-650 nanometers and reflectance is defined herein as the process where a fraction of the radiant flux incident on a surface is returned into the same hemisphere whose base is the surface and which contains the incident radiation (see Handbook of Optics, 2nd ed., McGraw-Hill, Inc., 1995). Often, the substrate is a polymeric material having refractive indexes in the range of 1.48 to 1.65, for example, polymethyl(meth)acrylate (PMMA) has a RI of 1.489; polycarbonate has a RI of 1.585; and polyethylene terephthalate (PET) has a RI of 1.64.
Known PSAs have RIs of about 1.47 or less. If these PSAs are used in optical applications, glare and reflectance may occur.
Therefore, the need exists for pressure-sensitive adhesives which have high refractive indexes.
The present invention provides pressure-sensitive adhesives which have a refractive index of at least 1.48. These pressure-sensitive adhesives are particularly suitable for optical applications where the substrate similarly has a high refractive index. The pressure-sensitive adhesives of the present invention advantageously allow for the matching of refractive index which reduces glare and reflectance.
The pressure-sensitive adhesives of the present invention comprise at least one monomer containing a substituted or an unsubstituted aromatic moiety.
One aspect of the present invention is a pressure-sensitive adhesive comprising the reaction product of: (a) at least one monomer selected from the group consisting of a monomeric acrylic or methacrylic acid ester of a non-tertiary alcohol, the alkyl group of which comprises from about 1 to about 12 carbon atoms, preferably from about 4 to about 8 carbons; and (b) at least one monomer containing a substituted or an unsubstituted aromatic moiety.
Another aspect of the present invention is a pressure-sensitive adhesive comprising the reaction product of: (b) at least one monomer containing a substituted or an unsubstituted aromatic moiety; and (c) at least one polar monomer copolymerizable with component (b).
Yet, another aspect of the present invention is a pressure-sensitive adhesive comprising the reaction product of: (a) at least one monomer selected from the group consisting of a monomeric acrylic or methacrylic acid ester of a non-tertiary alcohol, the alkyl group of which comprises from about 1 to about 12 carbon atoms, preferably from about 4 to about 8 carbons; (b) at least one monomer containing a substituted or unsubstituted aromatic moiety; and (c) at least one polar monomer copolymerizable with the monomer(s) of components (a) and (b).
The pressure-sensitive adhesives of the present invention may optionally comprise other monomers, crosslinkers, and additives.
Another embodiment of the present invention is a substrate coated with the pressure-sensitive adhesives of the present invention.
The present invention relates to pressure-sensitive adhesives having a refractive index of at least 1.48. Preferably, the pressure-sensitive adhesives have a refractive index of at least 1.50.
The pressure-sensitive adhesives of the present invention have a high refractive index and yet have a good balance of the four properties relevant for pressure-sensitive adhesives: adhesion, cohesion, stretchiness, and elasticity.
Refractive index is defined herein as the absolute refractive index of a material (e.g., a monomer) which is understood to be the ratio of the speed of electromagnetic radiation in free space to the speed of the radiation in that material, with the radiation being of sodium yellow light at a wavelength of about 583.9 nanometers (nm). The refractive index can be measured using known methods and is generally measured using an Abbe Refractometer.
The pressure-sensitive adhesives of the present invention are acrylate adhesives comprising at least one aromatic monomer which is either substituted or unsubstituted. The pressure-sensitive adhesives may further comprise at least one acrylic monomer selected from the group consisting of a monomeric acrylic or methacrylic acid ester of a non-tertiary alcohol and/or at least one polar monomer. The pressure-sensitive adhesives of the present invention optionally comprise other monomers which may be added to improve the properties of the adhesives, such as crosslinkers, and other additives such as tackifiers or plasticizers.
The acrylic monomers useful in the pressure-sensitive adhesive of the present invention are typically present at ranges from about 0 to about 93 parts by weight. Useful acrylic monomers include at least one monomer selected from the group consisting of a monomeric acrylic or methacrylic acid ester of a non-tertiary alkyl alcohol, the alkyl group of which comprises from about 1 to about 12 carbon atoms, preferably from about 4 to about 8 carbon atoms, and mixtures thereof.
Suitable acrylic monomers include, but are not limited to, those selected from the group consisting of the esters of acrylic acid or methacrylic acid with non-tertiary alkyl alcohols such as 1-butanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 1-methyl-1-butanol, 1-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol, 2-ethyl-1-butanol, 2-ethyl-1-hexanol, 3,5,5-trimethyl-1-hexanol, 3-heptanol, 2-octanol, 1-decanol, 1-dodecanol, and the like, and mixtures thereof. Such monomeric acrylic or methacrylic esters are known in the art and are commercially available.
The following aromatic monomers are high refractive index acrylic monomers, preferably all of which have homopolymer glass transition temperatures at or below 50xc2x0 C. These aromatic monomers, when polymerized alone or in the presence of other acrylic monomers, result in PSAs having RIs higher than are otherwise available. By adjusting the ratio of monomers, it is possible to make PSAs having RIs of at least 1.48.
The aromatic monomers of the present invention are represented by the following general Formula (I): 
wherein:
Ar is an aromatic group which is unsubstituted or substituted with a substituent selected from the group consisting of Bry and (R3)z 
wherein y represents the number of bromine substituents attached to the aromatic group and is an integer from 0 to 3;
R3 is a straight or branched alkyl of 2 to 12 carbons; and
z represents the number of R3 substituents attached to the aromatic ring and is an integer from 0 to 1,
provided that both y and z are not zero;
X is either oxygen or sulfur;
n is 0 to 3, preferably n is 0 or 1;
R1 is an unsubstituted straight or branched alkyl linking group of 2 to 12 carbons, preferably 2 to 8 carbons; and
R2 is either H or CH3.
In one embodiment of aromatic monomers, X is oxygen. Within this embodiment of aromatic monomers, a group of monomers includes those of Formula (II) wherein Ar is naphthyl: 
and R1, R2, and n are as defined above. The naphthyl group is unsubstituted or substituted as described above. Within the group of naphthyl aromatic monomers, another group is that wherein Ar is 1-napthyl or 2-napthyl.
Within the embodiment of aromatic monomers where X is oxygen, another group of monomers includes those of Formula (III) wherein Ar is phenyl: 
and R1, R2, and n are as defined above. The phenyl group is unsubstituted or substituted as described above. Within the substituted group of phenyl aromatic monomers, preferably the phenyl is dibromo substituted. Within the bromine substituted group, the phenyl monomers may also be 2-alkyl substituted or 4-alkyl substituted.
In an additional embodiment of aromatic monomers, X is sulfur. Within this embodiment of aromatic monomers, a group of monomers includes those of Formula (IV) wherein Ar is naphthyl: 
and R1, R2, and n are as defined above. The naphthyl group is unsubstituted or substituted as described above. Within the group of naphthyl aromatic monomers, an additional group is that wherein Ar is 1-napthyl or 2-napthyl.
Within the embodiment of aromatic monomers where X is sulfur, another group of monomers includes those of Formula (V) wherein Ar is phenyl: 
and R1, R2, and n are as defined above. The phenyl group is unsubstituted or substituted as described above. Within this group of phenyl aromatic monomers, preferably the phenyl is dibromo substituted. In another group, the phenyl monomers may be 2-alkyl substituted or 4-alkyl substituted.
Specific examples of aromatic monomers suitable in the present invention include, but are not limited to, 6-(4,6-dibromo-2-isopropyl phenoxy)-1-hexyl acrylate, 6-(4,6-dibromo-2-sec-butyl phenoxy)-1-hexyl acrylate, 2,6-dibromo-4-nonylphenyl acrylate, 2,6-dibromo-4-dodecyl phenyl acrylate, 2-(1-naphthyloxy)-1-ethyl acrylate, 2-(2-naphthyloxy)-1-ethyl acrylate, 6-(1-naphthyloxy)-1-hexyl acrylate, 6-(2-naphthyloxy)-1-hexyl acrylate, 8-(1-naphthyloxy)-1-octyl acrylate, 8-(2-naphthyloxy)-1-octyl acrylate, 2-phenylthio-1-ethyl acrylate, and phenoxy ethyl acrylate.
Polar monomers can be used to increase the cohesive strength of the pressure-sensitive adhesive. Generally, polar monomers are typically present at ranges from about 0 to about 12 parts by weight, preferably from about 2 to about 8 parts by weight. Useful polar monomers include, but are not limited to, those selected from the group consisting of ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, and ethylenically unsaturated phosphoric acids, and mixtures thereof. Examples of such compounds include, but are not limited to, those selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, citraconic acid, maleic acid, B-carboxyethyl acrylate, sulfoethyl methacrylate, and the like, and mixtures thereof.
Other useful copolymerizable polar monomers include, but are not limited to, acrylamides, N,N-dialkyl substituted acrylamides, N-vinyl lactams, and N,N-dialkylaminoalkyl acrylates, and mixtures thereof. Illustrative examples include, but are not limited to, those selected from the group consisting of N,N-dimethyl acrylamide, N,N-dimethyl methacrylamide, N,N-diethyl acrylamide, N,N-diethyl methacrylamide, N,N-dimethylaminoethyl methacrylate, N,N-dimethylaminopropyl methacrylate, N,N-dimethylaminoethyl acrylate, N,N-dimethylaminopropyl acrylate, and the like, and mixtures thereof.
Preferred polar monomers include acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile, and mixtures thereof.
In order to increase the shear or cohesive strength of the PSAs, a crosslinking additive may be incorporated into the PSA.
Two main types of crosslinking additives are commonly used. The first crosslinking additive is a thermal crosslinking additive such as a multifunctional aziridine. One example is 1, 1xe2x80x2-(1,3-phenylene dicarbonyl)-bis-(2-methylaziridine) (CAS No. 7652-64-4), referred to herein as xe2x80x9cBisamidexe2x80x9d. Such chemical crosslinkers can be added into solvent-based PSAs after polymerization and activated by heat during oven drying of the coated adhesive.
In another embodiment, chemical crosslinkers which rely upon free radicals to carry out the crosslinking reaction may be employed. Reagents such as, for example, peroxides serve as a source of free radicals. When heated sufficiently, these precursors will generate free radicals which bring about a crosslinking reaction of the polymer. A common free radical generating reagent is benzoyl peroxide. Free radical generators are required only in small quantities, but generally require higher temperatures to complete a crosslinking reaction than those required for the bisamide reagent.
The second type of chemical crosslinker is a photosensitive crosslinker which is activated by high intensity ultraviolet (UV) light. Two common photosensitive crosslinkers used for hot melt acrylic PSAs are benzophenone and copolymerizable aromatic ketone monomers as described in U.S. Pat. No. 4,737,559. Another photocrosslinker, which can be post-added to the solution polymer and activated by UV light is a triazine, for example, 2,4-bis(trichloromethyl)-6-(4-methoxy-pheynl)-s-triazine. These crosslinkers are activated by UV light generated from artificial sources such as medium pressure mercury lamps or a UV blacklight.
Hydrolyzable, free-radically copolymerizable crosslinkers, such as monoethylenically unsaturated mono-, di-, and trialkoxy silane compounds including, but not limited to, methacryloxypropyltrimethoxysilane (available from Gelest, Inc., Tullytown, Pa.), vinyldimethylethoxysilane, vinylmethyldiethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriphenoxysilane, and the like, are also useful crosslinking agents.
Multi-functional acrylates are useful for bulk or emulsion polymerization. Examples of useful multi-functional acrylate crosslinking agents include, but are not limited to, diacrylates, triacrylates, and tetraacrylates, such as 1,6-hexanediol diacrylate, poly(ethylene glycol) diacrylates, polybutadiene diacrylate, polyurethane diacrylates, and propoxylated glycerin triacrylate, and mixtures thereof.
Crosslinker is typically present from 0 to about 1 part by weight based on 100 parts by weight adhesive solids.
Crosslinking may also be achieved using high energy electromagnetic radiation such as gamma or e-beam radiation. In this case, no crosslinker may be required.
The present invention may optionally further comprise a chain transfer agent. Examples of useful chain transfer agents include, but are not limited to, those selected from the group consisting of carbon tetrabromide, mercaptans, alcohols, and mixtures thereof.
Other monomers may be added to improve performance, reduce cost, etc. in quantities which do not render the pressure-sensitive adhesive non-tacky. Examples of such other monomers include vinyl esters, vinyl acetate, 2-hydroxyethyl acrylate, styrene, and the like.
Following copolymerization, other additives may be blended with the resultant acrylate or methacrylate copolymer. For example, compatible tackifiers and/or plasticizers may be added to aid in optimizing the ultimate tack and peel properties of the PSA. The use of such tack-modifiers is common in the art, as is described in the Handbook of Pressure-Sensitive Adhesive Technology, edited by Donatas Satas (1982). Examples of useful tackifiers include, but are not limited to, rosin, rosin derivatives, polyterpene resins, coumarone-indene resins, and the like. Plasticizers which may be added to the adhesive of the invention may be selected from a wide variety of commercially available materials. In each case, the added plasticizer must be compatible with the PSA. Representative plasticizers include polyoxyethylene aryl ether, dialkyl adipate, 2-ethylhexyl diphenyl phosphate, t-butylphenyl diphenyl phosphate, di(2-ethylhexyl) adipate, toluenesulfonamide, dipropylene glycol dibenzoate, polyethylene glycol dibenzoate, polyoxypropylene aryl ether, dibutoxyethoxyethyl formal, and dibutoxyethoxyethyl adipate. When used, tackifiers are preferably added in an amount not to exceed about 150 parts by weight per 100 parts by weight copolymer, and plasticizer may be added in an amount up to about 50 parts by weight per 100 parts by weight copolymer.
Adhesives useful in this invention can be polymerized by conventional free-radical polymerization methods. Suitable methods of polymerization include solution polymerization, suspension polymerization, emulsion polymerization, and bulk polymerization.
The PSAs of the present invention may be coated upon a variety of flexible and inflexible backing materials using conventional coating techniques to produce PSA-coated sheet materials. Flexible substrates are defined herein as any material which is conventionally utilized as a tape backing or may be of any other flexible material. Examples include, but are not limited to, paper, plastic films such as polypropylene, polyethylene, polyvinyl chloride, polyester (polyethylene terephthalate), polycarbonate, polymethyl(meth)acrylate (PMMA), cellulose acetate, cellulose triacetate, and ethyl cellulose. Additionally, flexible substrates include, but are not limited to, woven fabric formed of threads of synthetic or natural materials such as cotton, nylon, rayon, glass, or ceramic material, or they may be nonwoven fabric such as air-laid webs or natural or synthetic fibers or blends of these. Examples of inflexible substrates include, but are not limited to, metal, metallized polymeric film, or ceramic sheet material. The PSA-coated sheet materials may take the form of any article conventionally known to be utilized with PSA compositions such as labels, tapes, signs, covers, marking indices, and the like.
The PSAs of the present invention may be coated using a variety of conventional coating techniques such as roll coating, knife coating, or curtain coating. The PSAs may also be coated without modification by extrusion, coextrusion, or hot melt techniques by employing suitable conventional coating devices. Primers may be used, but they are not always necessary. The resultant coatings do not require curing or crosslinking. However, if enhancement of resistance to solvents, etc., is desired, crosslinking may be effected by standard methods well-known in the art, such as radiation curing (electron beam or ultraviolet light) or chemical crosslinking.